fix(session): preserve tagged error messages (#18165)

packages/opencode/src/session/message-v2.ts

@@ -956,7 +956,7 @@ export namespace MessageV2 {
           { cause: e },
         ).toObject()
       case e instanceof Error:
-        return new NamedError.Unknown({ message: e.toString() }, { cause: e }).toObject()
+        return new NamedError.Unknown({ message: e instanceof Error ? e.message : String(e) }, { cause: e }).toObject()
       default:
         try {
           const parsed = ProviderError.parseStreamError(e)

packages/opencode/src/session/processor.ts

@@ -210,7 +210,7 @@ export namespace SessionProcessor {
                       state: {
                         status: "error",
                         input: value.input ?? match.state.input,
-                        error: (value.error as any).toString(),
+                        error: value.error instanceof Error ? value.error.message : String(value.error),
                         time: {
                           start: match.state.time.start,
                           end: Date.now(),

packages/opencode/test/session/message-v2.test.ts

@@ -4,6 +4,7 @@ import { MessageV2 } from "../../src/session/message-v2"
 import type { Provider } from "../../src/provider/provider"
 import { ModelID, ProviderID } from "../../src/provider/schema"
 import { SessionID, MessageID, PartID } from "../../src/session/schema"
+import { Question } from "../../src/question"
 
 const sessionID = SessionID.make("session")
 const providerID = ProviderID.make("test")
@@ -915,4 +916,15 @@ describe("session.message-v2.fromError", () => {
       },
     })
   })
+
+  test("serializes tagged errors with their message", () => {
+    const result = MessageV2.fromError(new Question.RejectedError(), { providerID })
+
+    expect(result).toStrictEqual({
+      name: "UnknownError",
+      data: {
+        message: "The user dismissed this question",
+      },
+    })
+  })
 })